Added controls to prevent resending alarms

- Removed old measures to acheive the same task; they did not appear
  to work and were based on the user rather than the alarm. Also, it
  depended on keeping and searching through a log of users sent to.
  This increases the computational complexity a bit, and also raises
  the question of when such a log should be cleared.
- Used the field_sent field of the alarm content type in order to
  track whether a given alarm has been sent or not. This way
  we eliminate the need to track users.